1. Field of the Invention
The present invention relates to a touch type electrophoretic display apparatus, especially to a touch type electrophoretic display apparatus using photo sensors for touch detection, wherein the output signal levels of the photo sensors can be adjusted according to the intensity of environmental light.
2. Description of the Related Art
Please refer to FIG. 1, which illustrates a sectional view of a prior art touch type electrophoretic display apparatus. As illustrated in FIG. 1, the touch type electrophoretic display apparatus includes a display substrate 100 and an electrophoretic display layer 110.
The display substrate 100 has a plurality of pixel driving circuits and a plurality of photo sensing circuits. The pixel driving circuit includes a pixel switch and a pixel capacitor, while the photo sensing circuit includes a photo transistor, a light intensity storing capacitor, and an output switch. The pixel driving circuits are used for generating an array of driving voltages to drive the electrophoretic display layer 110 to display a picture. The photo sensing circuits are used for outputting a plurality of photo sensing signals to a controller for detecting the coordinate of a touch event occurring in a touch plane.
When a finger of a user touches a local area of the touch plane, one of the photo sensing circuits will change the voltage level of the photo sensing signal, so that the controller can generate a touch coordinate accordingly. Regarding the circuit diagrams of the pixel driving circuit and the photo sensing circuit of the display substrate 100, please refer to FIG. 2. As illustrated in FIG. 2, the pixel driving circuit includes a transistor 201 and a pixel capacitor 202, while the photo sensing circuit includes a photo transistor 203, a light intensity storing capacitor 204, and an output switch 205.
The transistor 201 is used as the pixel switch, for storing a source voltage VS on the pixel capacitor 202 under the control of a gate voltage VG, to determine the grey level of a pixel of the electrophoretic display layer 110.
The photo transistor 203—under the bias of a turn-off voltage VOFF and a supply voltage VP—is used for generating a channel current according to an incident light, the stronger the incident light, the larger the channel current. When a touch event takes place at a position above one of the photo transistors 203, the incident light will become dimmer, and the channel current will reduce to a low level.
The light intensity storing capacitor 204 is used for integrating the channel current to generate a photo sensing voltage. The photo sensing voltage will exhibit a first level when the incident light on the photo transistor 203 is not shielded; the photo sensing voltage will exhibit a second level when the incident light on the photo transistor 203 becomes dimmer due to a touch operation of a user, wherein the first level is higher than the second level.
The output switch 205 is implemented by a transistor, used for outputting the photo sensing voltage through an output signal SOUT, under the control of an output scan voltage VSCAN. When the level of the output signal SOUT is lower than a threshold voltage, which is set between the first level and the second level, it indicates touched status; when the level of the output signal SOUT is higher than the threshold voltage, it indicates untouched status.
However, when environmental light is dim, the first level—corresponding to the situation that the incident light on the photo transistor 203 is not shielded—can be lower than the threshold voltage, and this will mistake untouched status as touched status. To solve this problem, one solution is to increase the channel width/length ratio of the photo transistors 203. Nevertheless, with the channel width/length ratio of the photo transistors 203 increased, the second level—corresponding to the situation that the incident light on the photo transistor 203 is shielded—can be higher than the threshold voltage when environmental light is strong, and this will mistake touched status as untouched status. That is, the first level and the second level can be shifted by environmental light to an extent that the touch status will not be determined correctly.
In view of the problems mentioned above, the present invention proposes a novel touch type electrophoretic display apparatus, which can adaptively adjust the level of the photo sensing voltage according to the intensity of environmental light, to prevent the touch status from being misjudged.